Due to the increased demand for higher capacity in voice, data and video transmission the communications market is expanding. In particular optical fiber communications technology has been developing in response to the market requirements. Optical transmitters are one type of fiber communications technology that is evolving to meet the increased demand.
Optical transmitters that utilize pre-distortion devices for distortion cancellation are well known. Typically, these transmitters are designed around a Mach-Zehnder optical modulator. The modulator is fed from a high power laser. The laser operates in the cool white mode and provides the “light source” that has its intensity or amplitude modulated in the Mach-Zehnder device.
The optical modulation is accomplished by feeding a radio frequency (RF) modulating signal to the appropriate port of the modulator. In this way RF amplitude modulation is converted to optical amplitude modulation.
A detrimental characteristic of the optical modulator is that its optical output/RF input transfer characteristic is very non-linear; it is sinusoidal in nature. Consequently for a large modulation index (the ratio of the peak variation actually used to the maximum design variation (e.g., that variation whereby the instantaneous amplitude of the modulated carrier reaches zero) severe odd order distortion is generated. In order to overcome this distortion an external means is required to compensate for the nonlinear transfer function.
Pre-distorters have been used to in the past to minimize odd order distortions generated in the modulator. These odd order distortions are reduced by a circuit that generates its own RF distortions and then injects them into the modulator out of phase with those that are generated by the modulator. The pre-distorters have been limited in their operating bandwidth and in their absolute distortion cancellation due to the use of magnetic components used to achieve the phase inversions that operate to cancel the distortion products.
Typically, an optical transmitter also includes some circuitry to monitor the distortions introduced in the transmitter. It has been common practice to use one or more auxiliary pilot tones along with the many main carrier signals and to monitor distortion products from the pilot tones to asses the operation of the transmitter. One drawback of this technique is that it uses additional, unwanted signals. Further, these signals produce distortion products with very low energy, making the signals difficult to pick up.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved mechanism to control distortion in a non-linear device, e.g., an optical transmitter.